1. Field of the Invention
The present invention relates to color registration and, more particularly, to color registration in multi-head printers.
2. Related Art
Various kinds of printers are well-known in the computing and digital image arts. Such printers include, for example dot-matrix printers, laser printers, inkjet printers and thermal printers. The focus of the present discussion is on thermal printers, so-named because they use thermal energy (heat) to produce printed output. More specifically, thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment from a donor sheet to the output medium or by initiating a color-forming reaction in the output medium. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
Conventional color thermal printers typically include multiple thermal print heads, each of which is responsible for printing a distinct color. In a four-head printer, for example, the four print heads may be responsible for printing cyan, magenta, yellow, and black, respectively. The print heads are typically are spaced some distance apart from each other in a row or other configuration.
The medium on which output is to be printed (referred to as the “output medium,” “web,” or “receiver”) typically is provided on a continuous roll, referred to as the “receiver roll.” The receiver is pulled from the receiver roll through the printer by a drive capstan roller located after the final print head. In this manner the receiver passes by and makes contact with each print head in succession. Each print head transfers pigment of a corresponding color from a donor element to the receiver as the receiver passes by it. In this way, a four color image may be printed by successively printing each of four single-color layers on the output medium. The processes of printing distinct colors of an image at successive print stations is referred to as “tandem printing.”
Printing a single four-color image in this manner requires that the image layers be in precise registration (alignment) with each other. The “registration” of multiple layers (and of individual dots within them) refers to the relative position between the layers. Ideally, all layers in an image are superimposed exactly on (i.e., precisely registered with) each other. Even a slight misregistration may cause noticeable visual artifacts, thereby detracting from the perceived quality of the resulting image.
Misregistration may be caused by any of a variety of factors. For example, although in an ideal printer the receiver moves through the printer at a constant speed, in practice the speed of the receiver may vary. Such variations in speed, if not properly taken into account, may cause a particular print head to print some or all of an image at the wrong location on the receiver, causing misregistration and other problems. For example, variation in the speed of the receiver while a print head is printing may cause the image layer being printed either to be compressed (if the receiver slows down) or stretched (if the receiver speeds up) on the receiver. Although such a distortion may not be objectionable in an image printed by a single print head, multiple such distortions superimposed on each other by multiple print heads can cause problems such as color variations in what should be areas of uniform color.
Various attempts have been made to ensure proper registration among the various layers of an output image by correcting for variations in receiver speed. For example, registration marks may be printed along the lateral edges of the output medium. Optical sensors positioned at each print head may read the registration marks to enable the printer to continuously recalculate the correct printing position for each layer of the image to be printed, thereby allowing the printer to compensate for shifting and stretching of the image on the output medium that may occur at or between each print head. An integral relationship may be established between the circumference of two output capstan drive rollers and the distance between successive print heads. Improved techniques for tracking the receiver web are disclosed in the commonly-owned and co-pending patent application entitled “Registration Error Reduction in a Tandem Printer.”
Techniques for correcting misregistration typically rely on the dimensions of the print heads remaining fixed over time. Misregistration may still even when such techniques are used, therefore, if the dimensions of two or more print heads change due to factors such as temperature, web stretch, mechanical deformation, or print head wear.
What is needed, therefore, are improved techniques for correcting for registration errors in tandem printers.